1. Field of the Invention
The present invention relates to an azeotropic composition (or an azeotropic mixture) of hexafluoropropylene dimer and acetonitrile and a separation processes by distillation utilizing the azeotropic composition. For example, the hexafluoropropylene dimer can be used as a heating medium for a high temperature, or a solvent, and also as a starting material for producing, for example various industrial surfactants such as a water/oil-repellent.
2. Description of the Related Art
A production process of the hexafluoropropylene dimer has been known.
The processes for selectively obtaining hexafluoropropylene dimer are disclosed in Japanese Patent Kokai Publication Nos. 49-134614 (A) and 51-125307 (A), in which, there is provided, as an example, a reaction process for the selective production of hexafluoropropylene dimer in which acetonitrile is used as a medium (or solvent). Mutual solubility of the hexafluoropropylene dimer and acetonitrile is limited, and thus, in the reaction wherein acetonitrile is used as the medium, liquid-liquid separation (namely phase separation into two liquid phases) occurs: one liquid phase is an acetonitrile phase containing a small amount of hexafluoropropylene dimer; and the other liquid phase is a hexafluoropropylene dimer phase containing a small amount of acetonitrile. There is an advantage of such a reaction in that the formed hexafluoropropylene dimer is not converted to a hexafluoropropylene trimer because a large amount of the formed hexafluoropropylene dimer is separated from the acetonitrile phase, i.e. transferred outside of the reaction system.
It is noted that in the reaction, two kinds of the following hexafluoropropylene dimers are synthesized:
(1) (CF.sub.3).sub.2 CFCF.dbd.CFCF.sub.3 (of which normal boiling point is 48.degree. C.); and PA1 (2) (CF.sub.3).sub.2 C.dbd.CFCF.sub.2 CF.sub.3 (of which normal boiling point is 51.degree. C.).
Hereinafter, the compound of formula (1) is also referred as "hexafluoropropylene dimer (1)" or simply "dimer (1)", and the compound of formula (2) is also referred as "hexafluoropropylene dimer (2)" or simply "dimer (2)." Further, hereinafter the above two compounds are also generically referred as "hexafluoropropylene dimer" or simply "dimer."
The hexafluoropropylene dimer phase obtained by the liquid-liquid separation in the above reaction does not provide the dimer of which purity is high because it contains a small amount of acetonitrile. Japanese Patent Kokai Publication No. 49-134614 (A) discloses a process for production of the dimer, comprising the step of adding water to a reaction mixture containing the dimer and acetonitrile as main components, which is followed by the liquid-liquid separation and distillation. However, the process is disadvantageous to the industrial process since acetonitrile as the solvent contains water which must be removed for reuse thereof.